It is known to manufacture rigid PUR foams using readily volatile alkanes and hydro-fluoro alkanes as blowing agents. The range of polyols is generally restricted in this case due to the low solubility of the above-mentioned blowing agents in most commercially available polyols and polyol mixtures. There are admittedly some polyol types with good solubility; trimethylolpropane- and ethylenediamine-started polyethers can be mentioned in particular here. In the case of trimethylolpropane ethers, the low functionality has a disadvantageous effect on the mold retention time (curing of the foams obtained), while with higher concentrations of ethylenediamine-started polyethers an excessively high inherent activity of the foam formulation is obtained.